Combustion effluents are a major source of air pollution. One type of particularly troublesome pollutant found in many combustion effluent streams is nitrogen oxides (NO.sub.x). Concern over NO.sub.x emissions is based on known cases of human health problems and environmental damage caused by NO.sub.x or compounds derived from NO.sub.x . NO.sub.x contributes to the formation of photochemical oxidants, commonly known as smog. Hydrocarbons react with NO.sub.x in the presence of sunlight to form these oxidants, which can have severe effects on the respiratory system. NO.sub.x can also be converted into nitric acid, one of the two principle components of acid rain.
Some of the major sources of NO.sub.x generated by man are stationary installations such as gas and oil fired steam boilers for electric power plants, process heaters, incinerators, coal fired utility boilers, glass making furnaces, cement kilns, oil field steam generators, gas turbines, and the like.
Various methods have been developed for reducing the concentration of NO.sub.x in combustion effluents. Such methods include the use of ammonia injection, catalytic conversion, as well as new designs and methods of operating combustion devices, such as burners. With regard to burner design and methods of operating burners to reduce NO.sub.x emissions, staged combustion has proven successful on a commercial scale for moderate reduction of NO.sub.x emissions.
In staged combustion, an oxygen-containing gas, particularly air, is typically supplied to the burner in the initial combustion stage at less than the amount needed to completely burn the fuel. Use of less than a stoichiometric amount of oxygen to fuel in the initial stage reduces NO.sub.x emissions due to the lack of oxygen required for NO.sub.x formation. The second stage combustion then completes the burning by introducing additional oxygen-containing gas into the combustion gases of the first combustion stage.
In conventional two-stage combustion, the second stage final combustion temperature usually approaches the same temperature as that of a one-stage combustion process, since only a relatively minor amount of heat is removed due to some uncontrolled passive heat transfer to the surrounding burner environment. Temperature is also an important factor in thermal NO.sub.x formation, since higher temperatures generate more NO.sub.x.
Application of NO.sub.x Control Techniques To Industrial Boilers, M. P. Heap et al. Air Pollutants--NO.sub.x and Particulate Emissions, AlChe Symposium Series, No. 175, Vol. 74, pp 115 to 125, 1978; and Controlling Nitrogen Oxides, Research Seminar, EPA-600/8-80-004, and Combustion, 2nd edition, Irvin Glassman, Academic Press, 1987 discuss two-stage combustion processes.
Further, U.S. Pat. Nos. 3,832,122; 3,837,788; 3,955,909; 4,013,399; and 4,050,877 teach air pollution control systems wherein the emission of NO.sub.x is reduced whereby less than a stoichiometric amount of air is fed into a primary combustion chamber so that the resulting combustion gases contain significant proportions of unburnt hydrocarbons and carbon monoxide due to incomplete combustion. This is then followed by injection of a regulated secondary air supply into the gases exiting the primary combustion chamber in order to complete combustion of all unburnt fuel in a secondary combustion zone.
Also, U.S. Pat. Nos. 3,873,67I; 3,911,083; 4,033,725; and 4,244,325 teach variations of a process for disposing of oxides of nitrogen by injecting said oxides into a combustion chamber where they are mixed with an excess of combustible products generated by burning a hydrocarbon fuel with less than its stoichiometric requirements of oxygen at a temperature from about 1200.degree. F. to 2000.degree. F. The oxides of nitrogen are thus reduced and safely vented to the atmosphere.
While such methods of combustion may have met with commercial success, there still exists a need in the art for improved methods of combustion which can further reduce NO.sub.x emission levels.